In the preparation of polymers, processes have been utilized to substantially reduce residual monomer levels. For instance, it is known to purify a polymer solution/emulsion on a powder bed at high temperature, thereby removing the solvent from the precipitating polymer. The monomer level is reduced by post-polymerizing unreacted monomer.
Precipitation polymerization of N-vinyl formamide (NVF) is known and disclosed in U.S. Pat. No. 4,421,602 issued to F. Brunnmueller et al on Dec. 20, 1983, U.S. Pat. No. 4,623,699 issued to F. Brunnmueller et al on Nov. 18, 1986 and U.S. Pat. No. 5,478,553 issued to R. Chandran et al on Dec. 26, 1995. These patents disclose the precipitation polymerization of N-vinyl formamide (NVF) in solvents such as methanol, ethanol, n- or iso-propanol, t-butanol, acetone and methyl ethyl ketone. The use of toluene, or mixtures of propanol and toluene is also known. However, it is expected that a relatively hydrophobic solvent like toluene would lead to a polymer containing high levels of formamide, a non-polymerizable contaminant that is present in the NVF monomer.
Precipitation polymerization of NVF in methanol is unsuccessful due to the coagulation of the methanol-swollen particles. One approach to circumventing this problem is to polymerize NVF in methanol with the assistance of poly(2-ethyl oxazoline) as a dispersing agent. While this approach may circumvent this problem, it also necessitates the use of a foreign component, which generally is not desired.
In methods of polymerizing NVF described above, there is no mention of methods specifically for removing non-polymerizable contaminants which may be present in the resultant polymer as an artifact of the monomers and/or processes used to prepare the monomers and/or polymers. Such contaminants may include, for example, raw materials used to prepare the monomers, such as formamide in the case of polymers prepared from NVF, intermediates which are formed during manufacture of the monomers and degradation by-products of the manufacture of the monomer. Additionally, non-polymerizable contaminants may be introduced via raw materials used to prepare the polymers, such as the monomers themselves, chain transfer agents, initiators and the like.
Precipitation polymerization of N-vinyl formamide is also disclosed in an article entitled "Synthesis of Amine Functional Homopolymers with N-Ethenylformamide" by R. J. Badesso et al in Adv. Chem. Ser., 1996, (Hydrophilic Polymers), pp. 489-504. This article shows polymerization of N-ethenylformamide by precipitation in iso-propanol followed by washing in iso-propanol or precipitation in t-butanol followed by washing in t-butanol. However, there is no mention in this article of reducing non-polymerizable contaminants in the product.
Polymers having significantly reduced levels of non-polymerizable contaminants particularly are desired and/or required for end-use applications such as cosmetics for application to skin, hair care products and pharmaceuticals. Therefore, it would be advantageous to develop a precipitation process which not only results in polymers having low residual monomer levels, but also having low non-polymerizable contaminant levels.
The precipitation process of the present invention does not require the use of stabilizers such as poly(2-ethyl oxazoline) and results in polymers which not only have reduced residual monomer levels, but just as important, exhibit significant reduction in the levels of non-polymerizable contaminants such as those discussed herein.